Bottom hole completion system for an intermittent plunger

ABSTRACT

A bottom hole assembly for an intermittent lift plunger system includes a retrievable downhole anchor which may be set and reset in one downhole trip, a retrievable packing device, and a plunger stop.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 11/163,540 filed on Oct. 21, 2005 entitled “Bottom HoleCompletion System for an Intermittent Plunger”, now U.S. Pat. No.7,347,273.

FIELD OF THE INVENTION

The present invention relates to a bottom hole completion system for aplunger lift system for intermittently lifting well fluids in an oil andgas well to the surface.

BACKGROUND OF THE INVENTION

Conventional pump systems for delivery of a fluid from a well boreinclude pump jacks or positive cavity pumps. While these pump systemshave achieved extensive use, they suffer from many disadvantages. Onedisadvantage is that these systems are expensive. This is particularlyproblematic for wells with low delivery rates as the cost of theequipment may be difficult to justify. Further, these systems requirethe use of external power or fuel, which requires the delivery of poweror fuel to the well site. Again, the cost of providing power to a wellhaving low delivery rate may be difficult to justify, particularly inremote well locations.

Differential gas pressure operated pistons, also known as plungers, havebeen used in producing subterranean wells where the natural wellpressure is insufficient to produce a free flow of gas, and especiallyliquids, to the well surface. A completed well typically includestubulars placed inside the well conduit, which extend from the reservoirof the well to the surface. The cylindrical plunger typically travelswithin the tubulars between the bottom hole assembly and the top of thetubulars, where a well valve and a lubricator are positioned. A springis typically included inside the lubricator assembly to absorb theimpact energy of the plunger when it reaches the surface. The well isshut in for a selected time period which allows downhole pressure tobuild up, then the well is opened for a selected period of time. Whenthe well valve is opened, the plunger is able to move up the tubulars,pushing a liquid slug to the well surface. When the well valve is laterclosed, the plunger, aided by gravity, falls downwardly to the bottom ofthe tubulars. Typically, the open and closed times for the well valveare managed by a programmable electronic controller.

When the plunger is functioning properly, fluids accumulate and stayabove the plunger and pressurized gases and/or fluids below the plungerare blocked from flowing up, around, and through the plunger. As aresult, the plunger and accumulated fluids are pushed upwardly. Theprior art devices use a variety of external, and sometimes internal,sealing elements which allow the plungers to block the upward flow ofgases and to slidingly and sealably engage the tubulars, whichaccomplishes the lifting of fluids to the surface depending upon thevariable well pressures. Improvements of this technology may permiteconomic operation of wells which were previously uneconomic.

The bottom hole assembly is run in a well using conventional wirelinetechniques and set immediately above perforations in the well casing, inorder to isolate the hydrostatic head from the producing zone. However,a conventional bottom hole assembly with a velocity tube requires alanding nipple. If a landing nipple is located far above theperforations, the velocity tube becomes too lengthy and restrictive.Also, placement in the landing nipple becomes difficult with older wellsthat may have tubing suffering from corrosion or scale.

Therefore, there is a continuing need in the art for an improved bottomhole assembly for a plunger system, which obviates or mitigatesdisadvantages in the prior art.

SUMMARY OF THE INVENTION

The present invention comprises a bottom hole assembly for a plungersystem for intermittently lifting fluids from a well. The bottom holeassembly comprises a downhole anchor and a packing element.

In one aspect, the invention comprises a retrievable bottom hole anchorcomprising:

-   -   (a) a hollow cylindrical mandrel having a lower end and an upper        end having a cone, said upper end adapted to accept a wireline        running or pulling tool;    -   (b) a J-slot sub engaging the lower end of the mandrel, said sub        defining a patterned slot having a running position, a pre-set        position, a set position, and a retrieving position;    -   (c) a slip assembly comprising:        -   i. a lower housing having an upper end comprising a slip            cage and a lower end comprising a bearing housing,            concentrically disposed around the J-slot sub;        -   ii. means for frictionally engaging the tubing;        -   iii. at least one pin disposed between the lower housing and            the J-slot sub, which pin engages the slot defined by the            J-slot sub;        -   iv. at least two slip arms each having an upper end and            lower end, wherein the lower end is retained by the slip            cage and the upper end comprises a tubing engaging slip,            said slip arms moveable between a retracted position and an            extended position wherein each said slip disposed to slide            over the cone when the slip assembly is actuated;    -   (d) wherein the mandrel and J-slot is moveable relative to the        slip assembly between a running position wherein the pin is        located in the running position in the slot and the slip arms        are in the retracted position, a set position where the pin is        located in the set position in the slot and the slips arms are        in the extended position, a pre-set position intermediate the        running position and the set position, and a retrieving position        where the pin is located in the retrieving position in the slot        and the slips arms are retracted.    -   (e) and wherein the anchor may move between the running, set and        extended positions by lifting and dropping the J-slot sub        relative to the pin.

In another aspect, the invention may comprise a downhole packing devicefor use in a bottom hole assembly comprising an anchor device asdescribed herein, said packing device comprising:

-   -   (a) a tubular mandrel having exterior ratchet pawls and a        plurality of collet fingers having an enlarged end;    -   (b) an upper housing comprising a top sub, a ratchet sub and a        ratchet ring, said upper housing axially moveable relative to        the mandrel, wherein said upper housing defines a plurality of        openings for accepting shear screws affixed to the mandrel;    -   (c) a deformable resilient seal concentrically disposed around        the mandrel, below the upper housing;    -   (d) a lower housing disposed below the seal comprising a release        sub having an internal shoulder defining a collet trap which        engage the collet finger ends;    -   (e) a collet locking tube concentrically disposed within the        mandrel, and moveable between a first locking position which        traps the collet fingers in the collet trap, and a second        release position, said collet locking tube having interior        ratchet teeth;    -   (f) wherein the upper housing is moveable between a first        extended position where the upper housing does not bear on the        seal and the shear screws are intact, and a second seal        position, where the upper housing bears on the seal, deforming        the seal radially outwards, and the shear screws are not intact;    -   (g) and wherein the mandrel pawls and the ratchet sub and        ratchet ring cooperate to maintain the upper housing in the seal        position as long as the collet fingers are trapped in the collet        trap.

Preferably, the packing device is adapted to engage a retrieving pullingpost and collet, wherein the retrieving collet fits within the colletlocking tube and includes pawls adapted to engage the collet lockingtube teeth, such that the retrieving pulling post and collet may be usedto pull the collet locking tube to its second release position.

In yet another aspect, the invention may comprise a method of setting adownhole anchor as claimed in claim 1, using a wireline toolstring,comprising the steps of:

-   -   (a) running the anchor downhole in a running position, to an        initial depth;    -   (b) lifting the toolstring to index the anchor to a pre-set        position;    -   (c) lowering the toolstring to index the anchor to a set        position;    -   (d) testing the ability of the anchor to support the toolstring        weight;    -   (e) repeating steps (a) to (c) in another location either uphole        or downhole from the initial depth if the anchor fails to        support the toolstring weight in the initial depth; and    -   (f) wherein steps (a) to (e) are performed in a single trip        downhole.

Preferably, the downhole anchor is combined with a packing device asdescribed herein, and the method includes the additional step of settingthe packing device by jarring the tool string to shear the shear pins isperformed after step (e) during the same trip downhole as steps (a) to(e).

The method includes the further steps of:

-   -   (a) landing a plunger stop and collet latch onto the top of the        anchor or the packing device;    -   (b) operating an intermittent lift plunger;    -   (c) pulling the plunger stop and collet latch; and    -   (d) inserting a toolstring having a pulling pole and collet into        the packing device mandrel, and pulling upwards to releasing the        packing device and to index the anchor to the pull position,        either in a single motion or two separate motions;    -   (e) retrieving the anchor and packing device to the surface.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of an exemplary embodimentwith reference to the accompanying simplified, diagrammatic,not-to-scale drawings. In the drawings:

FIG. 1 is a schematic view of a subterranean bottom hole assembly for anintermittent plunger.

FIG. 2 is a cross-sectional view of one embodiment of a downhole anchor.

FIG. 2A is a detailed view of the bearing assembly shown in FIG. 2.

FIG. 3 is a pictoral view of a J-slot sub of a downhole anchor of thepresent invention.

FIG. 4 is schematic of the slot pattern of a J-slot sub.

FIG. 5 is a cross-sectional view of the downhole anchor shown in FIG. 2in the pulling position.

FIG. 6 is a cross-sectional view of the downhole anchor shown in FIG. 2in the running position.

FIG. 7 is a cross-sectional view of the downhole anshor shown in FIG. 2in the maximum set position.

FIG. 8 is a cross-sectional view of a downhole anchor receiving aplunger stop and collet hatch.

FIG. 9A is a partial cross-sectional view of a collet hatch.

FIG. 9B is a partial cross-sectional view of an alternative embodimentof a collet hatch.

FIG. 10 is a partial cross-sectional view of a packing element of thepresent invention.

FIG. 11 is a partial cross-sectional view of a pulling pole andretrieving collet.

FIG. 12 is a cross-sectional view of a packing element and a pullingpole and retrieving collet in combination.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides for a bottom hole assembly for use withan intermittent plunger. When describing the present invention, allterms not defined herein have their common art-recognized meanings. Tothe extent that the following description is of a specific embodiment ora particular use of the invention, it is intended to be illustrativeonly, and not limiting of the claimed invention. The followingdescription is intended to cover all alternatives, modifications andequivalents that are included in the spirit and scope of the invention,as defined in the appended claims.

The bottom hole assembly and its components will be described withregard to its orientation in use, such that the longitudinal axis of thebottom hole assembly is substantially vertical. Therefore, the terms“lateral”, “radial” or “horizontal” shall refer to a direction or planesubstantially perpendicular to the longitudinal vertical axis of thecomponents of the bottom hole assembly (10).

As shown in FIG. 1, one embodiment of the bottom hole assembly (10)comprises a downhole anchor (12), optionally a packer (14), and avelocity tube (16), and a plunger stop (18). The anchor (12) functionsto fix the position of the plunger stop (18) used to activate a plunger(P) of the general type of plungers operated by differential gaspressure, as is well known in the art. An exemplary plunger is describedin Applicant's co-owned U.S. patent application Ser. No. 11/162,805,filed on Sep. 23, 2005, the contents of which are incorporated herein byreference. The plunger stop (18) is preferably positioned immediatelyabove a formation perforation.

The following description describes the installation of the componentsof the bottom hole assembly (10) into a production tubing string. Oneskilled in the art will realize that the same components may be adaptedto be installed in any string or continuous length of tubing (T), casing(C) or otherwise.

The packer (14) includes a radial sealing element which functions toseal the annulus between the tool string and the tubing wall, therebyensuring that well fluids are produced through the velocity tube (16).The plunger stop (18) includes a landing pin (19) and a spring (20),which serves to absorb the force when the plunger (P) lands on theplunger stop (18). The landing pin (19) causes a plunger valve to close,in the particular example illustrated. Once the valve closes, fluidpressure will begin to rise within the plunger internal chamber, causingplunger seals to expand outward. Once the seals expand to contact thewell bore surface, fluids will not be able to rise above the plunger (P)and the rate of change of the pressure differential will accelerate.Eventually, the pressure underneath the plunger (P) will overcome anyfrictional resistance of the seals against the tubing surface and thehydrostatic force of the fluid column above the plunger (P), and causethe plunger (P) to rise. Any fluids above the plunger (P) will thus belifted to the surface.

One embodiment of a downhole anchor (12) is shown in the Figures. Atubular mandrel (100) has an upper end (102) adapted as a fishneck and alower end attached to a J-slot sub (104). A lower housing (106)encircles the J-slot sub (104) and has a slip cage (108) at an upper endand attaches to a bearing housing (110) at a lower end. The lowerhousing (106) is fitted with a plurality of belly springs (112) whichare intended to create drag along the tubing (T) as the downhole anchor(12) is moved downhole. When fully relaxed, the springs (112) create adiameter greater than inside diameter of the tubing (T). Thus, whencompressed and inserted into the tubing (T), the belly springs (112)bear against the tubing (T), creating frictional drag as the downholeanchor (12) is moved within the tubing (T).

The upper portion of the tubular mandrel (100) is flared to create acone (114). A slip assembly includes plurality of slip arms (116)hingedly attached at one end to the slip cage (108), and have a tubingengaging surface, commonly referred to as a “slip” (118), at the upperend of each slip arm (116). Slips (118) are generally considered to bethe portion of a slip arm (116) having gripping teeth on the outside andan angle to match the cone (114) on the inside. The slip arms (116) areretained by a slip arm retaining ring (120) around the circumference ofthe tubular mandrel (100), which limits the outward movement of the sliparms (116), but permits sufficient outward movement to allow the slips(118) to contact the inside surface of the tubing (T). As the lowerhousing (106) slidingly engages the J-slot sub (104) and a lower portionof the mandrel (100), the slip assembly and the mandrel (100) may moveaxially relative to each other. As the slip arms (116) slide upwardsrelative to the mandrel (100), the cone (114) forces the slips (118)outwards and into contact with the tubing wall. O-ring seals (119) areprovided at the fishneck and between the mandrel (100) and the J-slotsub (104), and may also be included at the bottom of the tool to ensurepressure competence throughout the tool string.

As shown in FIG. 3, the J-slot sub (104) defines an exterior slot (122),which guides the movement of a pin (124) disposed between the bearinghousing (110) and the J-slot sub (104). The pin (124) is fixedvertically as part of the slip assembly, as shown in FIG. 2A. The pin(124) is vertically bounded by bearing rings (126) and by frictionreducing bearings (128), which may preferably be bronze bearings.

Accordingly, vertical movement of the J-slot sub (104) relative to thepin (124) (and thus the lower housing (106)) actuates movement of theslip assembly. The slot (122) is patterned to create at least threepositions for operation of the downhole anchor (12). The slot pattern isshown in FIG. 4 in a flat two-dimensional manner. As one skilled in theart will appreciate, the slot pattern is continuous around thecylindrical exterior surface of the J-slot sub (104) and may berepeated. In one embodiment, the slot pattern is repeated two or threetimes around the circumference of the J-slot sub (104), necessitatingthe use of two or three pins (124) respectively.

In a first running position (R), where the mandrel (100)/J-slot sub(104) is raised relative to the pin (124), the slip arms (116) areretracted. In a set position (S), where the mandrel (100)/J-slot sub(104) is lowered relative to the pin (124), the slip arms (116) areextended radially outward by the cone (114). In a preferred embodiment,a position intermediate the running (R) and set positions (S) isprovided as a pre-set position (PS). In a pull position (P), where themandrel (100)/J-slot sub (104) is again raised relative to the pin(124), the slip arms (116) are fully retracted, and the anchor (12) maybe pulled within the tubing (T) by fishing the mandrel (100). Thedownhole anchor (12) indexed to the pull position (P) is shown in FIG.5.

In operation, the anchor (12) may be lowered into the tubing stringusing conventional wireline techniques. The upper end (102) of themandrel (100) may be adapted to accept a standard wireline running andpulling tool and/or may also be adapted to accept the packer (14), asdescribed below. When the downhole anchor (12) is run into the well, itis indexed to the running position (R), where the pin (124) bearsupwards against the J-slot sub (104), as a consequence of the dragcreated by the belly springs (112) as the downhole anchor (12) is moveddownhole. As shown in FIG. 6, the slips (118) are partially retracted inthe running position, but do not interfere with the tubing (T).

Once in position, the tool string, which may include hydraulic jars,spang or mechanical jars and a weight bar, is raised to take up slack inthe tool string and index the downhole anchor (12) to the pre-setposition (PS). As may be seen, simply pulling up on the tool string willcause the pin (124) to move within the slot from the running position(R) to the pre-set position (PS). Lowering the tool string from thepre-set position (PS) will cause the pin (124) to move to the setposition (S), which actuates the slips (118) to engage the tubinginterior surface. FIG. 7 shows the slips (118) at a maximum setposition. The slips (118) may engage the tubing (T) before the slips(118) reach the maximum set position. At this point, the downhole anchor(12) should accept and hold the weight of the tool string. If not, thismay indicate that the downhole anchor (12) is placed in an unsuitableplace in the tubing (T), where the tubing (T) is damaged or corroded.The downhole anchor (12) may then be moved uphole or downhole to find asuitable anchor position.

In order to move up the hole, the tool string may be raised to index thedownhole anchor (12) to the pull position (P) and moved up the hole.Once in position, the tool string may be lowered to index the downholeanchor (12) to the running position (R), from where the set procedureabove may be repeated.

In order to move down the hole, after the tool string is lowered toindex the downhole anchor (12) to the running position, the tool stringmay lowered to a desired depth and the set procedure may be repeated.

The slot pattern (122) allows automatic indexing of the downhole anchor(12) between the various positions by simply raising and lowering thetool string, without lateral movement. Lateral movement within the slotpattern (122) is governed by the slot itself. The slot pattern (122) issuch that the downhole anchor (12) is indexed through its positionsautomatically and unidirectionally. In other words, the same series ofpositions results from continuously indexing of the downhole anchor(12). The path followed by the pin (124) within the groove is shown bythe arrows in FIG. 4. As a result, the downhole anchor (12) may be setand reset repeatedly within the tubing (T), without pulling thetoolstring and downhole anchor (12) out of the hole.

The plunger stop (18) may then be landed onto the downhole anchor (12)with a collet latch (130) shown in FIG. 8. The collet latch (130)includes a cage (132) defining a lip (134) which engages the profile onthe top of downhole anchor (12). The collet latch (130) is hollow andhas a plurality of oblique openings to allow fluids to pass throughrelatively unimpeded. The collet latch (130) may optionally include aone-way ball valve (136), to prevent reverse flow through the colletlatch (130), as is shown in FIG. 9B. A cage (138) retains the ball(136).

In some applications, it may be necessary to provide a seal inside thetubing string using a packer (14), one embodiment of which is shown inFIG. 10. The packer (14) and the downhole anchor (12) may be threadedtogether and inserted as a single unit by wireline. The packer (14)comprises a tubular mandrel (200) which slidingly engages an outerhousing which, in one embodiment, is made up of a top sub (202), anouter ratchet sub (204), a release sub (206), and a bottom sub (208).Downward movement of the top sub (202), outer ratchet sub (104), and theratchet ring (205) relative to the tubular mandrel (200) will squeezethe seal (210) radially outwards, creating a seal with the insidediameter of the tubing (T). A set screw (207) passes through a “split”in the ratchet ring (205) and engages a slot in the mandrel (200) inorder to rotationally lock the outer housing relative to the tubularmandrel (200).

In a running position (R), the packer (14) is an elongated position withthe seal (210) relaxed. The packer (14) is kept in this position by aplurality of shear screws (212) which pass through the top sub (202) andextend into the mandrel (200). Until sufficient force is brought to bearon the top sub (202) to break the shear screws (212), the packer (14) isheld in the running position (R). The bottom sub (208) is adapted to fitand engage the top of the downhole anchor (12). The packer (14) is setby jarring down with the tool string after the downhole anchor (12) hasbeen set. The downward impacts of the jarring motion shears the shearscrews (212) and allows the top sub (202), outer ratchet sub (204) andratchet ring (205) to move downwards along the mandrel (200). The pawls(214) on the mandrel (200) allow the ratchet sub (204) to ratchetdownwards to compress the seal (210). Collet fingers (216) on themandrel (200) are set in the collet trap (218) on the release sub (206),which locks the packer (14) in its set position (S). A collet lockingtube (220) having internal ratchet teeth is positioned within the colletfingers (216) and affixed to the bottom sub (208). The collet lockingtube (220) maintains the collet fingers (216) in the collet trap (218)of the release sub (206). Once the ratchet sub (204) has ratcheteddownwards on the mandrel (200), the seal (210) exerts an upward force onthe mandrel (200) and thus the collet fingers (216), which are locked inthe collet trap (218) by the collet locking tube (220).

Once the downhole anchor (12) and then the packer (14) are set, aplunger stop (18) as may then be landed onto the packer (14) with acollet latch (130), as shown schematically in FIG. 1 and in more detailin FIG. 8. The collet latch (130) includes a cage (132) defining a lip(134) which engages and latches (130) onto the profile on top of packer(14). The collet latch (130) preferably includes a ball check.

To release the packer (14), the plunger stop (18) and collet latch (130)are simply pulled out of the packer (14). The packer (14) and downholeanchor (12) are in set positions (S) and easily provide sufficientresistance to the pulling action. Once the plunger stop (18) and colletlatch (130) are removed, a retrieving collet (230) fitted to a pullingpost (232) is stabbed into the packer mandrel (200) until it landsinside the collet locking tube (220). The collet (230) has a cage (231)with exterior pawls (234) which engage the ratchet teeth on the colletlocking tube (220). On the inside of the cage (132), opposite the pawls(234), each cage member includes a collet lug such that a cage ridge(236) is formed. The inside diameter of the cage ridge (236) defined bythe collet lugs is less than the cage (132) itself. The pulling post(232) extends into the collet cage (132) and ends with an upset (238),which is an enlarged section on the end of the pulling post (232), andwhich has an outside diameter approximately equal to the inside diameterof the cage ridge (236). The pulling post (232) is moveable axiallywithin the cage (132) between a position where the pulling post upset(238) is aligned with the ridge (236), as shown in FIG. 11, and alowered position where the upset (238) is disposed just below the ridge(236). As is apparent, when the upset (238) and the ridge (236) arealigned, the cage (231) cannot deform inwardly.

Axial movement of the pulling post (232) is limited by a shear pin (233)fitted through an opening in the rod (232) and fixed to the collet(230). The shear pin (233) permits disengagement of the pulling post(232) by application of sufficient force to break the shear pin (233),which may be necessary in cases where the locking tube (220) cannot bereleased. Shearing the pin (233) moves the pulling post upset (238) frombelow the ridge (236) allows the fingers (216) to relax (flex inward)and pull free from the collet locking tube (220).

When the collet is stabbed into the packer (14), the protrusion andridge are not aligned, the collet cage (231) may flex inwards,permitting the pawls (234) to ratchet past the teeth on the colletlocking tube (220). When the pulling post (232) is pulled upwards, theprotrusion and ridge align, preventing the pawls (214) and the teethfrom disengaging, which then causes the collet locking tube (220) toslide upwards within the packer (14). Upwards movement of the colletlocking tube (220) releases the collet fingers (216) of the packermandrel (200) from the collet trap (218) of the release sub (206). Themandrel (200), top sub (202) and ratchet sub (204) thus move upwardsrelative to the release sub (206), thereby extending the packer (14) toan unset position, with the seal (210) retracted.

As will be apparent to those skilled in the art, various modifications,adaptations and variations of the foregoing specific disclosure can bemade without departing from the scope of the invention claimed herein.The various features and elements of the described invention may becombined in a manner different from the combinations described orclaimed herein, without departing from the scope of the invention.

What is claimed is:
 1. A retrievable bottom hole anchor comprising: (a)a hollow cylindrical mandrel having a lower end and an upper end havinga cone, said upper end adapted to accept a wireline running or pullingtool; (b) a J-slot sub engaging the lower end of the mandrel, said subdefining a patterned slot having a running position, a pre-set position,a set position, and a retrieving position; (c) a slip assemblycomprising: i. a lower housing having an upper end comprising a slipcage and a lower end comprising a housing, concentrically disposedaround the J-slot sub; ii. means for frictionally engaging the tubing;iii. at least one pin disposed between the lower housing and the J-slotsub, which pin engages the slot defined by the J-slot sub; iv. at leasttwo slip arms each having an upper end and lower end, wherein the lowerend is retained by the slip cage and the upper end comprises a tubingengaging slip, said slip arms moveable between a retracted position andan extended position wherein each said dog disposed to slide over thecone when the slip assembly is actuated; (d) wherein the mandrel andJ-slot sub is moveable relative to the slip assembly between a runningposition where the pin is located in the running position in the slotand the slip arms are in the retracted position, a set position wherethe pin is located in the set position in the slot and the slips armsare in the extended position, a pre-set position intermediate therunning position and the set position, and a retrieving position wherethe pin is located in the retrieving position in the slot and the slipsarms are retracted; (e) and wherein the anchor may move between therunning, set and extended positions by lifting and dropping the J-slotsub relative to the pin.
 2. The bottom hole anchor of claim 1 whereinthe slot pattern is repeated at least twice on the J-slot sub, and thepawl and bearing assembly comprises at least two pins, wherein each pinengages a separate slot pattern.
 3. The bottom hole anchor of claim 1wherein the means for frictionally engaging the tubing comprises atleast one belly spring.
 4. The bottom hole anchor of claim 2 wherein theslip assembly comprises a bearing ring comprising the at least two pinsand a rotary thrust bearing for limiting vertical movement of thebearing ring while reducing rotary friction.
 5. The bottom hole anchorof claim 5 wherein the rotary thrust bearing comprises a first bronzebearing placed above the bearing ring and a second bronze bearing placedbelow the bearing ring.